A wide variety of fibrous materials have been suggested as filters for cigarette smoke. Cellulose acetate tow is the most commonly used filter material. One disadvantage associated with this filter material is, however, that it is slow to degrade. Whilst most of the components of a spent smoking article dissociate into their individual constituent parts and degrade within a relatively short period of time when exposed to moisture and/or mechanical abrasion, cellulose acetate filter material is slow to degrade because the cellulose acetate fibres themselves are effectively not water soluble and therefore poorly biodegradable.
For disposable products, it is desirable to use materials which are biodegradable. Biodegradable polymers disposed of in bioactive environments degrade by the enzymatic action of microorganisms such as bacteria, fungi and algae. Their polymer chains may also be cleaved by non-enzymatic processes such as chemical hydrolysis. As used herein, the term “biodegradable” means that the composition degrades within one year using the standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions.
Poly(lactic acid) or polylactide (PLA), is an attractive biodegradable and biocompatible polymer. It is derived from renewable resources (e.g., corn, wheat, or rice) and it is biodegradable, recyclable, and compostable. In addition, PLA exhibits excellent processability. Actually, PLA has better thermal processability compared to other biodegradable materials such as poly(hydroxyalkanoates) (PHAs), poly(68 -caprolactone) (PCL), etc. It can be processed by injection moulding, film extrusion, blow moulding, thermoforming, fibre spinning, and film forming. However, the use of PLA can be restricted due to the fact that it is a hydrophobic polymer and is not able to solubilise or disperse in water.
It is desirable to produce biodegradable filter materials, preferably are made from materials which can be easily processed to produce fibres, which are thermally processable and which have good mechanical and physical properties.
CA may be treated with plasticizers for use in smoking article filters. This involves applying the plasticizer (usually in liquid form) to the surface of the CA fibres, for example by spraying the liquid plasticizer on to the CA tow. The plasticizer acts by binding adjacent fibres to one another at their contact points, thereby affording the filter rods sufficient hardness for cigarette manufacture and use. Thus, although the materials added to CA in this way are generally referred to as plasticizers, they are really acting as binders or hardeners rather than as plasticizers. Suitable plasticizers for this use include triacetin (glycerin triacetate), TEC (triethyl citrate) and PEG 400 (low molecular weight polyethylene glycol). Plasticized cellulose acetate tow is also known to improve the selective removal of semi-volatile compounds found in smoke (e.g. phenol, o-cresol, p-cresol and m-cresol). For this effect, it is considered to be necessary for the plasticizer to be present on the surface of the CA fibres. Unfortunately, the addition of a plasticizer which binds fibres actually can result in a reduction in the degradability of the filter material. The binding of the fibres certainly slows the separation of the individual fibres making up the tow in a spent smoking article, thus maintaining the bundle of fibres and reducing their exposure to the elements that will carry out any degradation process.
Because of the fibre-binding effect of plasticizers, conventional CA filters often include between 6 and 8% plasticizer. It has been found that including plasticizer in greater amounts than this has a detrimental effect on the cellulose acetate tow, causing holes to be formed.